Heretofore, it has been desired to provide silver halide photographic materials with a higher sensitivity. In particular, it has been keenly desired to provide spectrally sensitized silver halide photographic materials with a higher sensitivity.
A spectral sensitizing technique is an extremely important and indispensable technique in the preparation of a high sensitivity light-sensitive material which exhibits an excellent color reproducibility. A spectral sensitizer has an inherent effect of absorbing light in the long wavelength range that is not substantially absorbed by a silver halide photographic emulsion and transferring its light energy to the silver halide. Thus, the rise in the amount of light captured by the spectral sensitizer is favorable for enhancing photographic sensitivity. Accordingly, attempts to enhance the captured amount of light have been made by increasing the amount of a spectral sensitizer to be added to the silver halide emulsion. However, if the amount of the spectral sensitizer to be added to the silver halide emulsion exceeds its optimum value, it causes a great desensitization. This is a phenomenon normally called dye desensitization which occurs in the inherent sensitive wavelength range of silver halides where there is no light absorption by sensitizing dyes. When a great desensitization occurs, it gives a reduced overall sensitivity, despite the spectral sensitizing effect. In other words, the less dye desensitization is, the more is the sensitivity in the wavelength range of light absorption by the sensitizing dye (i.e., spectral sensitization). Accordingly, the reduction of dye desensitization is an important assignment in the spectral sensitizing technique. The longer the sensitive wavelength range of sensitizing dye is, the more is dye desensitization. This phenomenon is further described in T. H. James, "The Theory of the Photographic Process", pp. 265-268, Macmillan, 1966.
As described in Tadaaki Tani, "Journal of the Physical Chemistry", vol. 94, page 1298, 1990, it has been known that sensitizing dyes having a reduction potential of higher than -1.25 V show a low relative quantum yield of spectral sensitization. In order to enhance the relative quantum yield of spectral sensitization of such dyes, it has been proposed to effect supersensitization by capturing positive holes as described in the above cited "The Theory of the Photographic Process", pp. 259-265, 1966.
As the foregoing supersensitizer for eliminating desensitization there may be used a compound having a lower oxidation potential than sensitizing dyes. For example, U.S. Pat. Nos. 2,313,922, 2,075,046, 2,448,858, and 2,680,686, British Patent 1,230,449, and Belgian Patent 771,168 disclose styryl bases.
However, these styryl bases have an insufficient effect of providing a higher sensitivity. Further, these styryl bases are disadvantageous in that they are poor in storage stability.